Rapid kinetics of sweet taste transduction

The objective of this research was to characterize the roles of 3{dollar}spprime,5spprime{dollar}-cyclic adenosine monophosphate (cAMP), 3{dollar}spprime,5spprime{dollar}-cyclic guanine monophosphate (cGMP) and inositol 1,4,5-triphosphate (IP{dollar}sb3){dollar} in sucrose taste transduction using the blowfly, Phormia regina. Since the events that underlie sweet taste transduction occur within a behaviorally significant time of 100 msec, I employed electrophysiology and biochemistry quench flow techniques which measure physiological changes within the subsecond time frame.; In electrophysiological studies, taste sensilla were treated with Tris buffer, a partially purified extract of the taste modifier, Hodulcin (HDE), 3-isobutyl-1 methylxanthine (IBMX), or an HDE/IBMX mixture. Sensilla were stimulated with 50 mM sucrose for 5 sec at 1-min intervals over a 10-min period. The numbers of action potentials fired in the first 100 msec of responses to sucrose were counted and compared to those fired in response to an earlier Tris treatment from the same sensillum. In biochemical studies, homogenates of sensilla were presented with buffer, 50 mM sucrose or a sucrose/IBMX mixture for 0, 25, 50, 75, 100 and 200 msecs, and levels of cAMP, cGMP and IP{dollar}sb3{dollar} were measured.; The HDE initially suppressed action potential responses to sucrose, while IBMX suppressed responses at later time points. The HDE/IBMX mixture had an additive effect. Sucrose or sucrose/IBMX suppressed cAMP and cGMP below basal levels within the first 50 msecs of sucrose contact--a period of rapid receptor cell action potential firing. There was a greater magnitude of suppression of cAMP as compared to cGMP at all time points. The IP{dollar}sb3{dollar} levels did not change during the first 100 msec.; These studies suggest that increases in cyclic nucleotide(s) suppress receptor cell firing rates to sucrose. Furthermore, sucrose and sucrose/IBMX decreased cAMP and cGMP within the expected time frame for mediation of transduction. These results suggest that sucrose taste is transduced via a decrease in cAMP and/or cGMP in P. regina. The HDE-induced suppression of behavioral responses and decrease in receptor cell responses to sucrose may be mediated by increases in cAMP and/or cGMP.